Electroluminescent MIS structures incorporating Langmuir-Blodgett films
The Langmuir-Blodgett (LB) technique provides an excellent method of depositing thin, uniform, insulating films of accurately defined thickness. They can therefore be used effectively in the investigation of metal/thin insulator/ semiconductor electroluminescent structures where the insulator requirements are good dielectric properties allied to uniformity and an accurately defined thickness. The insertion of a fatty acid LB film into the gold/n-type GaP system produces an increase in the effective barrier height of the device and also enables electroluminescence to be observed. The electroluminescent efficiency is shown to depend on the thickness of the organic film whereas the increase in the effective barrier height is relatively independent of this parameter. The optimum efficiency is obtained for a film thickness of approximately 27 mm, well above that predicted on the basis of direct quantum mechanical tunnelling, The increase in the effective barrier height is shown not to be due to an increased band bending in the semiconductor and a simple energy band model, which accounts for most of the experimental observations, is proposed. Measurements made on the diodes under illumination both support the proposed model and provide information about the mechanism of minority carrier injection. Diodes fabricated using phthalocyanine as the LB film exhibit very different characteristics, in particular the optimum thickness for EL efficiency is approximately 5.6 mm and the diodes appear to conform to the conventional tunnel injection theory. The prospects for commercial exploitation are quite promising in the case of the phthalocyanine-based diodes, particularly if the system can be extended to incorporate an efficient II-VI phosphor as the luminescent material. The results of preliminary experiments using ZnSe as the semiconductor are encouraging in this respect. Preliminary results for two other potential applications of LB films in metal/insulator/semiconductor devices are also described. The first of these concerns an attempt to invert the surface of p-type GaAs, with a view to producing an n-channel inversion mode field effect transistor, and the second describes the high field injection of charge into silicon dioxide, which has potential applications in the field of semiconductor memory devices.